A key requirement on Long Term Evolution (LTE) in 3GPP Wireless Communications Systems is frequency flexibility for transmissions between a radio base station and a mobile terminal over a radio link. For this purpose, carrier bandwidths between 1.4 MHz and 20 MHz are supported, as is both Frequency Division Duplex (FDD) and Time Division Duplex (TDD), so that both paired and unpaired frequency spectrum can be used. For FDD, the downlink, i.e. the link from a base station to a mobile terminal, and uplink, i.e. the link from a mobile terminal to a base station, use different frequencies so called “paired frequency spectrum” and can hence transmit simultaneously. For TDD, uplink and downlink use the same frequency “unpaired frequency spectrum” and cannot transmit simultaneously. Uplink and downlink can however share the time in a flexible way, and by allocating different amounts of time, such as the number of subframes of a radio frame, to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in uplink and downlink.
The above asymmetry also leads to a significant difference between FDD and TDD. Whereas for FDD, the same number of uplink and downlink subframes is available during a radio frame, for TDD the number of uplink and downlink subframes may be different. In LTE time is structured into radio frames of 10 ms duration, and each radio frame is further divided into 10 subframes of 1 ms each. One of many consequences of this is that in FDD, a mobile terminal can always send feedback in response to a data packet in an uplink subframe subject to a certain fixed processing delay. In other words, every downlink subframe can be associated to a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e. to each uplink subframe is associated exactly one downlink subframe. For TDD however, since the number of uplink and downlink subframes during a radio frame may be different, it is in general not possible to construct such one-to-one association. For the typical case with more downlink subframes than uplink sub-frames, it is rather so that feedback from several downlink subframes requires to be transmitted in each uplink subframe.
In LTE, a radio frame of 10 ms duration is divided into ten subframes, wherein each subframe is 1 ms long. In case of TDD, a subframe is either assigned to uplink or downlink, i.e. uplink and downlink transmission cannot occur at the same time. Furthermore, each 10 ms radio frame is divided into two half-frames of 5 ms duration where each half-frame consists of five subframes as shown in FIG. 1a. 
The first subframe of a radio frame is always allocated to downlink transmission. The second subframe is split into three special fields, Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS), with a total duration of 1 ms.
UpPTS is used for uplink transmission of sounding reference signals and, if so configured, reception of a shorter random access preamble. No data or control signaling can be transmitted in UpPTS.
GP is used to create a guard period between periods of downlink and uplink subframes and may be configured to have different lengths in order to avoid interference between uplink and downlink transmissions and is typically chosen based on the supported cell radius. Thus a large cell may benefit from a longer guard period as the signal propagation time becomes longer for signals sent over longer distances.
DwPTS is used for downlink transmission much like any other downlink subframe with the difference that it has shorter duration.
Different allocations of the remaining subframes to uplink and downlink transmission are supported, both allocations with 5 ms periodicity in which the first and second half-frame have identical structure, and allocations with 10 ms periodicity for which the half-frames are organized differently. For certain configurations, the entire second half-frame is assigned to downlink transmission. Currently supported configurations use 5 ms periodicity as illustrated in FIG. 1b and 10 ms periodicity as depicted in FIG. 1c. In case of 5 ms periodicity, the ratio between downlink and uplink may e.g. be 2/3, 3/2, 4/1, etc. In case of 10 ms periodicity, the ratio between downlink and uplink may e.g. be 5/5, 7/3, 8/2, 9/1 etc.
In the downlink of LTE, Orthogonal Frequency Division Multiplex (OFDM) with a subcarrier spacing of 15 kHz is used. In the frequency dimension the subcarriers are grouped into resource blocks, each containing twelve consecutive subcarriers. The number of resource blocks depends on the system bandwidth, and the minimum bandwidth corresponds to six resource blocks. Depending on the configured cyclic prefix length, a 1 ms subframe contains either 12 or 14 OFDM symbols in time. The term resource block is also used to refer to the two-dimensional structure of all OFDM symbols within a half subframe. The special downlink subframe DwPTS has a variable duration, and can assume lengths of 3, 9, 10, 11 or 12 OFDM symbols for the case with normal cyclic prefix, and 3, 8, 9 or 10 symbols for the case with extended cyclic prefix.
In the uplink of LTE, Single-Carrier Frequency-Division Multiple Access (SC-FDMA), also referred to as Discrete Fourier Transform (DFT)-pre-coded OFDM is used. The underlying two-dimensional (time and frequency) numerology is the same in terms of subcarrier spacing, cyclic prefix lengths and number of OFDM symbols. The major difference is that modulated data symbols to be transmitted in certain OFDM symbols are subject to a DFT and the outputs of the DFT are mapped to the subcarriers.
In order to improve performance of transmission in both the downlink and uplink direction, LTE uses Hybrid Automatic Repeat Request (HARQ). The basic idea of HARQ, for downlink transmission, is that after receiving data in a downlink subframe the terminal attempts to decode it and then reports to the base station whether the decoding was successful by sending an acknowledgement (ACK) or unsuccessful by sending a negative acknowledgement (NAK). In the latter case of an unsuccessful decoding attempt, the base station thus receives a NAK in a later uplink subframe, and can retransmit the erroneously received data.
Downlink transmissions are dynamically scheduled, i.e. in each subframe the base station transmits control information on which terminals are to receive data and upon which resources in the current downlink subframe. Such a control information message to a terminal is referred to as a downlink assignment. A downlink assignment thus contains information to the terminal about in which resources a subsequent data will be sent, and also information necessary for the terminal to decode the subsequent data, such as modulation and coding scheme. By resources is here meant some set of resource blocks. This control signaling is transmitted in the first 1, 2 or 3 OFDM symbols in each subframe. The data sent to a terminal in a single downlink subframe is often referred to as a transport block.
A terminal may thus listen to the control channel, and if it detects a downlink assignment addressed to itself, it may try to decode the subsequent data. It may also generate feedback in response to the transmission, in the form of an ACK or a NAK depending on whether the data was decoded correctly or not. Furthermore, from the control channel resources on which the assignment was transmitted by the base station, the terminal may determine the corresponding uplink control channel resource.
For LTE FDD the terminal may in response to a detected downlink assignment in subframe n, send an ACK/NAK report in uplink subframe n+4. For the case with so-called Multiple In Multiple Out (MIMO) multi-layer transmission, two transport blocks are transmitted in a single downlink subframe, and the terminal will respond with two ACK/NAK reports in the corresponding uplink subframe.
The assignment of resources to the terminals is handled by the scheduler, which takes into account traffic and radio conditions so as to use the resources efficiently while also meeting delay and rate requirements. Scheduling and control signaling may be done on a subframe to subframe basis. Currently there is no dependency between the downlink assignments sent in the different downlink subframes, i.e. each downlink subframe is scheduled independently of others.
As described above, the first step for a terminal to receive data from the base station in a downlink subframe is to detect a downlink assignment in the control field of a downlink subframe. In the case that the base station sends such an assignment but the terminal fails to decode it, the terminal obviously cannot know that is was scheduled and will hence not respond with an ACK/NAK in the uplink. This situation is referred to as a missed downlink assignment. The lack of acknowledgement is sometimes referred to as a Disrupted Transmission (DTX).
If the absence of an ACK/NAK can be detected by the base station, it can interpret such absence of an ACK/NAK as a missed downlink assignment which may initiate subsequent retransmissions. Typically the base station may at least retransmit the missing packet, but it may also adjust some other transmission parameters.
For FDD a terminal can always respond to a downlink data transmission with an ACK/NAK after a fixed delay of 4 subframes, while for TDD there is not a one-to-one relation between uplink and downlink subframes. This was discussed above. Thus the terminal cannot always send an ACK/NAK in response to a downlink assignment in subframe n in uplink subframe n+4, since this subframe may not be allocated to uplink transmission. Hence each downlink subframe may be associated with a certain uplink subframe subject to a minimum processing delay, meaning that ACK/NAKs in response to downlink assignments in subframe n are reported in subframe n+k with k>3. Furthermore, if the number of downlink subframes is larger than the number of uplink subframes, ACK/NAKs in response to assignments in multiple downlink subframes may need to be sent in a single uplink subframe. For a given uplink subframe, the number of associated downlink subframes depends on the configuration of subframes to uplink and downlink, and can be different for different uplink subframe configurations, as further illustrated in Table 1.
TABLE 1Sub frame index n0123456789UL:0DLDLUL 1UL 0UL 1DLDLUL 1UL 0UL 1DL1DLDLUL 2UL 1DLDLDLUL 2UL 1DLcon-2DLDLUL 4DLDLDLDLUL 4DLDLfigu-3DLDLUL 3UL 2UL 2DLDLDLDLDLration4DLDLUL 4UL 4DLDLDLDLDLDL5DLDLUL 9DLDLDLDLDLDLDL6DLDLUL 1UL 1UL 1DLDLUL 1UL 1DL
Table 1 illustrates the number of downlink subframes associated with each uplink subframe. Uplink subframes are marked UL, downlink subframes are marked DL.
Since downlink assignments can be given independently across downlink subframes, a terminal may be assigned downlink transmissions in multiple downlink subframes that are all to be acknowledged in a single uplink subframe. Thus the number of assigned downlink subframes may exceed the number of uplink subframes. Hence the uplink control signaling needs to support, in some way, feedback of ACK/NAKs from multiple downlink transmissions from a terminal in a given uplink subframe, such as is illustrated in FIG. 1d. In the example depicted in FIG. 1d, four ACK/NAKs in response to downlink transmission in four downlink subframes are to be reported in one single uplink subframe.
In the uplink, DFT-precoded OFDM, also referred to as SC-FDMA is used. A subframe contains two slots with 6 or 7 symbols per slot. In each slot, one symbol is used for transmission of demodulation reference signals and the other symbols may be used for data transmission and control transmission.
Data to be transmitted on the PUSCH is channel coded, scrambled, modulated and then divided into blocks of M symbols, where M is the number of subcarriers allocated in a slot. Each block of M symbols is then subject to a DFT and then, mapped to the carriers used in each slot.
Furthermore, when data is transmitted in the uplink on the PUSCH, control signaling such as ACK/NAK feedback replaces some of the data symbols, this since the control channel and data channel cannot be used simultaneously due to the single carrier property which is important to ensure good uplink coverage. This may be referred to as multiplexing data and control before the DFT and interpreted as a form of time multiplexing. When it comes to ACK/NAK feedback, the encoded ACK/NAK bits may simply replace the data in certain positions, typically close to the reference signals (RS) in order to achieve good performance also at high speeds which cause channel variations.
FIG. 1e illustrates multiplexing of data and ACK/NAK control on the Physical Uplink Control Channel (PUCCH) for a case with normal CP. A block of data is generated by mapping the output of a Fast Fourier Transform (FFT) of a block of modulated symbols to a set of subcarriers. In certain symbols, part of the data symbols are replaced by control information, such as encoded ACK/NAK bits before the corresponding DFT and mapping to subcarriers.
The number of bits, or symbols, taken from the data parts and allocated for transmission of ACK/NAK control information is determined from the modulation and coding scheme used for the data as well as a configurable offset. Hence, it is possible for the eNodeB to control the number of bits allocated for ACK/NAK transmission and the encoded ACK/NAK bits then simply overwrites the data in the corresponding positions.
When the terminal is to transmit a single bit of ACK/NAK feedback, it will encode the bit with 0 or 1 and use repetition coding to construct encoded sequences of appropriate lengths. The encoded ACK/NAK sequences are then scrambled and modulated so that two constellation points of maximum distance are used. Essentially, this means that the ACK/NAK effectively uses Binary Phase-Shift Keying (BPSK) modulation, also sometimes referred to as Phase Reversal Keying (PRK) whereas the other symbols may use Quadrature Phase-Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM) such as e.g. 16 QAM or 64 QAM.
When the terminal is to transmit two bits of ACK/NAK feedback, it will encode the two bits with a (3,2) simplex code and then use repetition of the coded bits to construct encoded sequences of appropriate length. The encoded sequences are then scrambled and modulated so that four constellation points with maximum Euclidean distance is used for the ACK/NAK transmission. Effectively this means that the ACK/NAK bits are transmitted using QPSK modulation whereas the data may be transmitted using QPSK, 16 QAM or 64 QAM modulation.
In short, when the terminal has detected downlink assignments for associated downlink sub-frames, it will generate ACK/NAK encoded sequence of lengths determined from the modulation and coding scheme and a configurable offset. It will then replace some of the data symbols with encoded ACK/NAK symbols. When there is no assignment and hence no ACK/NAK feedback, the terminal will use the corresponding resources for data transmission.
There is one case that requires some care, and that is when the terminal misses the downlink assignment. The base station will then expect that the terminal transmits an ACK/NAK whereas the terminal will transmit random data. The base station will therefore need to perform DTX detection to distinguish between random data and an ACK or NAK. The target error probability for DTX→ACK, i.e. the probability that data is interpreted as an ACK is around 1e-2, whereas the target probability that a terminal misses an assignment is around 1e-2 meaning that the probability that the terminal misses a packet and the eNodeB judges that the data is correctly received by estimating a received ACK is around 1e-4 which coincides with the target error rate of NAK to ACK i.e. the probability that a NAK is interpreted as an ACK.
The base station may thus expect an ACK/NAK in certain positions where data is transmitted. For this purpose, the base station performs DTX detection, in order to distinguish between random data and ACK or NAK.
DTX detection on PUSCH hence means that the base station needs to distinguish random data from an ACK or NAK. This may be done by letting the base station correlate the received signals with the different signal alternative for ACK (and NAK) and comparing with a threshold. For sufficiently large magnitude, an ACK or NAK may be declared. It requires that the length of the ACK/NAK-sequence is sufficiently long.
One obvious way to approach the above problem is to allow the terminal to transmit multiple individual ACK/NAK bits, for each downlink transmission, in a single uplink subframe. Such protocols have however worse coverage than transmission of one or two ACK/NAK reports. In addition, the more ACK/NAKs that are allowed to be transmitted from a single terminal, the more control channel resources need to be reserved in the uplink. To improve control signaling coverage and capacity, it is possible to perform some form of compression, or bundling, of ACK/NAKs. This means that all ACK/NAKs that are to be sent in a given uplink subframe are combined into a smaller number of bits, such as a single ACK/NAK report. As an example, the terminal can transmit an ACK only if the transport blocks of all the downlink subframes were received correctly and hence to be acknowledged. In any other case, meaning that at a NAK for at least one downlink subframe is to be transmitted, a combined NAK is sent for all downlink subframes. As described above, to each uplink subframe in TDD a set of downlink subframes can be associated rather than a single subframe as in FDD, for which downlink transmissions are to be given ACK/NAK response in the given uplink subframe. In the context of bundling this set is often referred to as the bundling window.
FIG. 1f and FIG. 1g illustrates two different uplink (UL):downlink (DL) allocations as an example for how bundling windows may be used. Uplink subframes contains an upward directed arrow, downlink subframes contains an downward directed arrow, and DwPTS/GP/UpPTS subframes comprises both a downward directed arrow and an upward directed arrow in FIGS. 1f and 1g. In the illustrated examples, the number of associated downlink subframes, K, is different for different subframes as well as for different asymmetries.
For the 4DL:1UL configuration in FIG. 1f, the uplink subframe in each half frame is associated with four downlink subframes, such that K=4.
For the 3DL:2UL configuration in FIG. 1g, the first uplink subframe in each half frame is associated with two downlink subframes, thus K=2, while the second is associated with a single DL subframe, K=1.
Another advantage of bundling is that it allows reusing the same control channel signaling formats as for FDD, independently of the TDD uplink/downlink asymmetry. The disadvantage is a loss in downlink efficiency. If the base station receives a NAK it cannot know how many and which downlink subframes were received erroneously and which were received correctly. Hence it may need to retransmit all of them.
A problem with ACK/NAK bundling is that a terminal may miss a downlink assignment, which may not be indicated in the bundled response. For instance, assume that the terminal was scheduled in two consecutive downlink subframes. In the first subframe the terminal misses the scheduling downlink assignment and will not be aware that it was scheduled, while in the second subframe it did successfully receive the data. The terminal will, as a result, transmit an ACK, which the base station will assume holds for both subframes, including data in subframe the terminal was not aware of. As a result, data will be lost.
The lost data needs to be handled by higher-layer protocols, which typically takes a longer time than HARQ retransmissions and is less efficient. In fact, a terminal will not transmit any ACK/NAK in a given uplink subframe only if it missed every downlink assignment that was sent during the bundling window associated with the uplink subframe.
For this reason, a Downlink Assignment Index (DAI) which represents the minimum number of previous and future assigned downlink subframes within the bundling window may be introduced. The terminal may, when receiving multiple downlink assignments, count the number of assignments and compare it with the signaled number in the DAI to see whether any downlink assignment has been missed. In the case that the scheduler is purely causal, the DAI only represents the number of previously assigned downlink subframes within the bundling window. For the case with ACK/NAK feedback on the uplink control channel PUCCH, which is used when there is no data to transmit in the uplink, the terminal may select a PUCCH feedback channel associated with last received/detected downlink assignment and in this way signal to the base station which was the last received downlink assignment. The base station may then detect if the terminal has missed any downlink assignments, in the end of the bundling window.
Alternatively, the base station scheduler may perform a partial scheduling of future downlink subframes within the bundling window and indicate to the terminal whether it will also receive one or more additional assignment in addition to the number of previously assigned subframes. Hence, the DAI then represents the number of previous assignments plus at least one more for the case that at least one more downlink subframe will be assigned. The terminal will then know by inspecting the DAI of the last received downlink assignment not only the number of previous subframes but also whether there will be at least one more. Hence, the DAI contain the sum of the previous assignments plus the minimum number of future assignments.
A third alternative, in addition to the two previously mentioned alternatives is to signal the total number of downlink subframes within the bundling window. The three mentioned alternative uses of the DAI are illustrated in FIG. 1h. 
An alternative solution to handle missed downlink assignments may be to signal in the uplink the number of received downlink assignments in addition to the bundled ACK/NAK. The base station, which has knowledge of the number of assigned downlink subframes, can then compare the reported number of subframes to judge whether the terminal has missed one or more assignments.
One candidate solution for multiple ACK/NAK transmission on PUCCH is to employ PUCCH resource selection. Each PUCCH format 1a or 1b resource can carry 1 or 2 bit of information with BPSK or QPSK modulation. Assuming that the terminal has received D downlink subframes and that associated with each received downlink subframe it can determine a PUCCH format 1b resource, which can carry 1 or 2 bits. Then in total the terminal can by selecting resource, and the bits carried on the resource signal, in total up to 4D different messages, assuming PUCCH format 1b with QPSK modulation. For PUCCH format 1a with BPSK modulation, there are up to 2D resources. Each such message can represent a combination of ACK/NAK/DTX for the D different subframes. With D=4, there are 16 messages which is enough to convey 4 bits of information representing for example ACK or NAK/DTX of four different subframes. In fact, 4D+1 signal alternatives are possible in total since an additional alternative is to not send anything at all from the terminal, i.e. a disrupted transmission DTX.
For PUSCH, there is currently no agreed solution.
Thus, a missed downlink assignment will in general result in block errors that need to be corrected by higher-layer protocols, which in turn has a negative impact on performance in terms of throughput and latency. Also, increasing the delay may cause undesirable interactions with Transmission Control Protocol (TCP) based applications.
To be able to handle all error case for ACK/NAK bundling, in particular when the bundled ACK/NAK is transmitted on PUSCH, the scheduler needs to account for future assignments within the bundling window. This may however be challenging, from a scheduler implementation view, and may bring a latency increase. This since scheduling of not only one subframe, but at least partially also one future subframe, requires more processing time and also access to HARQ feedback which may not be available. A preferred solution is thus to use the DAI so that it only contains a counter of the number of previous assigned subframes.
When it comes to ACK/NAK bundling, there is a problematic case, namely when the bundled ACK/NAK is to be transmitted on the data channel PUSCH, time multiplexed with the data. The terminal can then not indicate to the terminal by means of selecting a PUCCH channel for the ACK/NAK which was the last received downlink assignment. Hence, the scheduling may then be non-causal in the sense that the DAI contain information on future assignments.
When it comes to multiplexing of multiple ACK/NAKs, a problem is that currently only feedback of 1 and 2 bits of ACK/NAK feedback is defined and there is no solution for more than three bits.